These teachings relate to optical systems in which the range of radiances for performing non-uniformity correction or radiometric calibration of the sensor is greatly increased.
Current detector technologies, particularly those operating in the infrared portion of the spectrum, often have significant amounts of non-uniformity, which cause the imagery generated by imaging and hyperspectral imaging sensors to be non-uniform in their output. Current approaches to the correction, or flattening, of this output imagery is accomplished through various non-uniformity correction, more commonly known as NUC, methods, where images of uniform scenes at different radiance levels are used to calculate the response of each pixel in the imagery, and then calculate an array of transformation terms to match the response of each pixel to a uniform average.
For example, in a two-point NUC approach, one would typically expose the sensor to two separate uniform radiance sources and then use the output imagery to calculate an array of individual linear fits to the data. These linear fit terms would then be used to calculate a transform matrix of gain and offset terms, which when applied to the output imagery of the sensor, would result in a uniform image when exposed to those particular radiance sources.
However, since the response of each pixel is typically non-linear to some degree, the true response of the array will depart from the linear mapping of NUC, resulting in spatial noise in the imagery, commonly known as residual fixed pattern noise, or FPN, or RFPN. This additional noise adds to the temporal and system noise of the sensor, reducing its overall sensitivity and detection capability. The further the radiance level the source is from the radiance levels used to calculate the NUC terms, the more FPN that is typically present in the imagery. As a result, it is a common practice to use radiance levels that bracket the range of radiances expected from the scene to minimize the impact of this FPN.
In the visible spectrum, widely spaced radiance levels can be accomplished through the use of a lamp source and a shutter, which can provide both a sufficiently bright source and a very dark source to bracket the radiance levels of a scene. This is not, however, easily accomplished in the longwave infrared, or LWIR, spectrum, where the radiance sources are thermal in nature. Achieving a high radiance source can be accomplished with a warm, high emissivity, blackbody target. The high emissivity reduces any reflected sources from other targets and provides higher radiance levels at lower temperatures. Achieving a low radiance source, however, is more difficult. For example, the radiance output in the LWIR for a target with a temperature of 0° C. is only half that of a target with a temperature of 40° C. While objects in the scene may be bounded by temperatures over this range, their emissivity can range from very low to very high, resulting in a range of radiance values in the scene from equally very low to very high. To reduce the impact of FNP, there is a need for low radiance sources, and unfortunately reducing the emissivity of a radiance target only increases the amount of transmitted or reflected radiance from sources outside of the target, resulting in inaccurate knowledge of the target radiance as well as eliminating the benefit of the lower emissivity. Operating the radiance source at temperatures below 0° C. presents its own problems in that they cannot be used in the presence of air due to condensation and potential frost on the surface of the source that corrupts its radiance output.
There is therefore a need for a device to provide lower radiance sources for non-uniformity correction than that which is currently available.